Plasma-treated Bombyx mori cocoon separators for high-performance and sustainable lithium-ion batteries
Graphical abstract
Introduction
Owing to their high energy density, high coulombic efficiency, low self-discharge features, quasi zero-memory, high open circuit voltage, and long lifespan, lithium-ion batteries (LIBs) represent promising energy storage devices which, as recognized by the 2019 Nobel Prize in Chemistry [1], stand at the forefront of the on-going technological revolution [[2], [3], [4], [5], [6], [7]]. LIBs have made a tremendous impact on our society. Used in many popular devices, such as the ubiquitous smartphones, laptop computers, digital cameras, and tablets, they may be scaled to power cars, or miniaturized to power pacemakers.
The separator is a core component of LIBs [[8], [9], [10]]. It is placed between the cathode and the anode as a physical barrier, to ensure electronic insulation, and hinder short-circuit problems, while enabling ion transport. Although it is a non-electrochemically active component, the role of the separator is far from passive, influencing markedly the battery cost, life, reliability, and safety. During LIB operation, high values of Li+ ion conductivity and Li+ ion transference number for the electrolyte in the separator pore spaces are essential to avoid deleterious degradation processes associated with heat generation and lithium plating. On the other hand, the ionic transport across the electrolyte-filled pore separator network must be homogeneous to mitigate the risks of incomplete lithiation/delithiation, and local overcharge. Lagadec et al. [9] highlighted that the separator structure (characterized by porosity, tortuosity, permeability, and connectivity), the separator chemistry (represented by the materials composition), as well as the separator/electrolyte interplay (measured by the electrolyte wettability) impact LIB performance.
Polyolefins have been the dominant separators of commercial LIBs in the past decades [11]. However, these polymers have major shortcomings that need to be tackled. They are hydrophobic materials offering poor compatibility with the liquid electrolyte. In addition, they undergo severe thermal shrinkage above 100 °C because of their low melting points. Besides, they have low porosity, and exhibit poor mechanical properties. Currently, significant efforts are being made to obviate these disadvantages and to meet the greater demands that new applications place on LIB technology. The quest for durable, reliable, and safe separators, produced by means of cost-effective processes, and combining high ionic conductivity and excellent thermal stability, is imperative. One pathway to optimize the technology of separators is to gather inspiration from nature. Natural polymers with multilength-scale hierarchical structures are particularly attractive, because they are renewable, biocompatible, biodegradable, and low cost.
In the light of this bio-inspired approach, silk attracted our attention. Out of the class of natural fibers, silks, with a tradition of more than 5,000 years in the textile industry, are deeply appreciated, mainly because of the lustrous appearance and soft touch of silk fabrics. Spun by certain arthropods (e.g. silkworms, spiders, scorpions, mites, bees, and flies), silks are fibrous materials possessing a series of extraordinary attributes. Their unique mechanical behavior (high tensile strength and great extensibility), which outperforms that of the best available synthetic materials, is correlated with the presence of hierarchical mesoscopic structures [12]. As the fibers are non-toxic, biocompatible [13], and have controllable biodegradability, silk is regarded as an excellent biomaterial candidate for healthcare applications [[14], [15], [16], [17]] and biological analysis [18]. Their heat-conducting ability, easy processing into films, and wide optical window have opened a wide range of potential new opportunities. Not surprisingly, a myriad of high-tech applications of silk-based materials in a variety of domains, spanning from lithography [19], optics [20,21], photonics [22], and electronics [[23], [24], [25]], to energy [[26], [27], [28], [29], [30], [31], [32]], have emerged in the last decade. The vast majority of the research on silk-based materials and devices relied on the use of silk fibroin (SF) extracted from Bombyx mori silkworm cocoons. For instance, we proposed SF-based separators for LIBs [31]. In the native fiber, SF is a water-insoluble fibrous core protein with more than 5,000 amino acids and high molecular weight (200–350 kDa or more), which is surrounded by silk sericin (SS), a water-soluble glue-like globular protein (10–300 kDa).
Owing to present safety and sustainability concerns, and to the pressing need to avoid the non-green and time/energy-consuming chemical procedures involved in the removal of SS from the raw silk fibers (degumming process [33]), a part of the scientific community has turned its attention to the native silk cocoons. Despite this challenging concept is still in infancy, several studies have successfully explored it and exciting future possibilities of application have been suggested [[34], [35], [36]]. We proposed that raw B. mori silkworm cocoons are suitable bio-inspired environmentally friendly separators for the design of the next-generation of LIBs with improved sustainability and enhanced safety [32]. We showed that silkworm cocoons are a valuable alternative to polyolefins, offering several merits for LIBs [32]: (1) high degree of porosity (85 ± 27%), ensuring electrolyte infiltration and Li+ flow; (2) thermal decomposition above ca. 280 °C; (3) good mechanical resistance; (4) maximum electrolyte uptake values of the order of 300–400%; (5) good electrolyte retention; (6) preservation of the physicochemical properties after battery operation; and (7) self-extinguishing ability, contributing to reduce or eliminate fire risk. A LIB incorporating a cocoon separator soaked in the ethylene carbonate (EC)/dimethyl carbonate (DMC)/lithium hexafluorophosphate (LiPF6) liquid electrolyte exhibited excellent cycling performance, with a discharge capacity of 86 mAh/g, and a capacity retention of 81% after 50 cycles at C-rate [32].
Motivated by the fact that plasma treatments applied over polyethylene membrane separators improved the performance of LIBs [37,38], and also driven by the fact that the exposure of silk-based materials to several plasmas [[39], [40], [41], [42], [43], [44], [45]] exerted an important effect on the surface hydrophilicity, we decided to expose B. mori silkworm cocoon separators to oxygen (O2) and nitrogen (N2) plasmas. The selection of these two reactive plasmas aimed at avoiding the introduction of different chemical species at the surface of the native protein fibers. Although both provide similar results, their use is in general dictated by the type of material to be treated or by the type of activation envisaged. Indeed, when they are applied, reactive functional groups including oxygen atoms (e.g. hydroxyl [–OH], carbonyl [CO], and carboxylic [–COOH] groups) and nitrogen atoms (e.g. amine groups [–NH2] groups), respectively, are formed at the surface of the material.
Herein, a comprehensive characterization of the surface of B. mori silkworm cocoon separators exposed to O2 and N2 plasmas at various treatment times has been carried out using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The impact of both plasmas on the morphology, wettability, wicking, and flammability of the materials has been evaluated. We demonstrate that the exposure of a raw cocoon separator to O2 plasma for just a few seconds significantly boosted its electrochemical features (EC/DMC/LiPF6 electrolyte uptake capacity, ionic conductivity, and interfacial properties). The performance of a cathodic half-cell with C–LiFePO4 assembled with the cocoon separator sample featuring the best properties was investigated. The results obtained are very exciting, because they outrival those reported for analogue battery systems including some commercial separators.
Section snippets
Materials
B. mori cocoons were obtained from APPACDM (Castelo Branco, Portugal). Poly(vinylidene fluoride) (PVdF, Solef 5130, Solvay), lithium iron phosphate/carbon (C–LiFePO4, Phostech Lithium), carbon black (Super P–C45, TIMCAL Graphite & Carbon), N-methyl-2-pyrrolidone (NMP, FLUKA, 99.9%), and hexane (Sigma-Aldrich, 99.9%) were used as received. Customized battery grade electrolyte 1 M solution of LiPF6 in EC:DMC (50:50 vol/vol) was purchased from Solvionic. The cocoon separators were dried at 60 °C
Impact of the plasma treatment on the physicochemical properties of the B. mori cocoon separators
The surface modification of polymer fibers (natural or synthetic) is of large relevance in the areas of textiles [47] and biomedical engineering [48], among others. Over the last few years, plasma treatments have gathered increasing attention, as they offer major advantages with respect to conventional wet processes in terms of environmental footprint and health concerns. Plasma treatments are fast, affordable, safe, reliable, and dry processes, which do not produce waste or contaminations, and
Conclusions
In the present work, we report a fast and eco-friendly surface modification of B. mori cocoon-based separators by two plasmas (O2 and N2) applied at different exposure times. On plasma exposure, the behavior of the separators changed dramatically from superhydrophobic to superhydrophilic. This effect was beneficial in terms of wettability and wicking, without jeopardizing the flammability behavior and thus safety issues.
In terms of physical impact, both plasma treatments caused marked
CRediT authorship contribution statement
Rui F. P. Pereira: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing. Renato Gonçalves: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing. Helena M. R. Gonçalves: Methodology, Formal analysis, Investigation, Writing - Original Draft. Daniela M. Correia: Investigation, Methodology, Writing - Original Draft. Carlos M. Costa:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by National funds by Foundation for Science and Technology (FCT) in the framework of the strategic Funding UID/FIS/04650/2019, UID/QUI/00686/2016, UID/QUI/00686/2018, and UID/QUI/00686/2019. The authors thank FEDER funds through the COMPETE 2020 Program and National Funds through FCT under the projects PTDC/FIS-MAC/28157/2017, PEst-OE/QUI/UI0616/2014, LUMECD (POCI-01-0145-FEDER-016884 and PTDC/CTM-NAN/0956/2014), UniRCell (SAICTPAC/0032/2015 and
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